A spark plug is widely used in an automotive internal-combustion engine to ignite an air-fuel mixture by a spark discharge. In response to the recent demand for high engine output and fuel efficiency, it is desired that the spark plug has an increased ignitability to exhibit a higher ignition-limit air-fuel ratio and to achieve proper lean mixture ignition and quick combustion.
Such a plasma-jet spark plug includes a center electrode and a ground electrode (external electrode), which is connected with a metal shell, defining a spark discharge gap therebetween, and an insulator (housing) made of ceramic or the like and surrounding the spark discharge gap so as to form a small discharge space, so-called a cavity (chamber). A spark discharge is generated through application of a high voltage between the center electrode and the ground electrode, and dielectric breakdown caused at this time enables to feed electric current with a relatively low voltage. Thus, a further energy supply causes a phase transition of the discharge to eject a plasma formed within the cavity from an opening portion (external electrode hole) called an orifice for ignition of an air-fuel mixture (e.g., see Patent Document 1 or 2).
A plasma-jet spark plug disclosed in Patent Document 1 or 2 has a cylindrical metal shell in which a front end portion thereof is closed to serve as a ground electrode and form an orifice in the center. Further, a front end face of the insulator accommodated in the external electrode comes in contact with an inner face of the ground electrode so that the orifice and the cavity are coaxially formed. In another form of the plasma-jet spark plug, the front end portion of the metal shell is joined to a separate ground electrode and define the orifice in the center of the ground electrode while the front end face of the insulator comes in contact to an inner face (inner side face) of the ground electrode (see Patent Document 1, FIG. 2).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H2-72577.
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2006-294257.
However, when an insulator and a metal shell is formed with a strict dimensional control in the manufacturing of a plasma-jet spark plug and a front end face of the insulator comes in contact with an inner face of the ground electrode as in the plasma-jet spark plug according Patent Document 1 or 2, the insulator can be damaged due to a difference in thermal expansion coefficient of the materials constituting the insulator, the metal shell and the ground electrode under the influence of thermal cycle at the time of use. On the other hand, when a large gap is formed between the front end face of the insulator and the inner face of the ground electrode resulting from a manufacturing tolerance, the plasma energy escapes into the gap, and the plasma is, therefore, not ejected into an intended direction, or the amount of plasma ejection (ejection length) is likely to decrease (be short) when the plasma formed within the cavity is ejected through the orifice. Although the insulator is securely accommodated in the metal shell by a crimping method, the insulator can be damaged due to a rise of internal stress when the front end face of the insulator is crimped while being strongly pressed to the inner face of the ground electrode resulting from a manufacturing tolerance of the insulator and the ground electrode.
The present invention is accomplished in view of the foregoing problems of the prior arts. An advantage of the present invention is to provide a plasma-jet spark plug in which an insulator and a ground electrode are disposed apart from each other in an axial direction so as to prevent a damage of the insulator, and the spark plug is capable of reducing an energy loss of the ejected plasma by defining a dimension of a clearance between the insulator and the ground electrode whereby a deterioration in an ignitability of the plasma-jet spark plug is prevented.